Method for eluting calcium from steelmaking slag, and method for collecting calcium from steelmaking slag

ABSTRACT

The purpose of the present invention is to provide a method for eluting Ca from steelmaking slag such that a large amount of Ca can be eluted from the steelmaking slag into an aqueous solution containing carbon dioxide. The present invention executes, in this order: a step for removing an iron-containing compound from steelmaking slag by performing magnetic separation on the steelmaking slag; and a step for bringing the steelmaking slag subjected to magnetic separation into contact with an aqueous solution containing carbon dioxide. In addition, the aqueous solution containing carbon dioxide and the steelmaking slag are brought into contact with each other while the steelmaking slag is being pulverized or the surface of the steelmaking slag is being ground.

TECHNICAL FIELD

The present invention relates to a method for eluting calcium fromsteelmaking slag, and a method for recovering calcium from steelmakingslag.

BACKGROUND ART

Steelmaking slag (such as converter slag, pretreatment slag, secondaryrefining slag and electric furnace slag) generated during a steelmakingprocess is used in various applications, for example, as cementmaterials, road base materials, construction materials and fertilizers(see NPLs 1 to 3). Some of steelmaking slag not used for the aboveapplications is landfilled.

Steelmaking slag is known to contain elements, such as calcium (Ca),iron (Fe), silicon (Si), manganese (Mn), magnesium (Mg), aluminum (Al),phosphorus (P), titanium (Ti), chrome (Cr) and sulfur (S). The elementcontained most in steelmaking slag is Ca which is used in a large amountduring a steelmaking process, and usually, Fe is the second mostcontained element. Typically, Ca and Fe occupy about 20 mass % to 50mass % and about 1 to 30 mass % of the total mass of steelmaking slag,respectively.

Steelmaking slag contains Ca in the forms of, for example, free lime (asremaining quicklime (CaO) which is fed during the steelmaking process,or quicklime precipitated during solidification of the steelmakingslag), calcium hydroxide (Ca(OH)₂) or calcium carbonate (CaCO₃) eachgenerated from the free lime reacting with water vapor or carbon dioxidein the air, and/or calcium silicate (such as Ca₂SiO₄ or Ca₃SiO₅) orcalcium iron aluminum oxide (Ca₂(Al_(1-X)Fe_(X))₂O₅) each generated fromcalcium reacting with, for example, Si or Al (hereinafter, the compoundscontaining calcium in steelmaking slag are also collectively referred toas “Ca compounds”).

Calcium carbonate and calcium oxide are major slag-forming materials inpig iron making process during an iron-making process and in a steelmaking process, and used as, for example, a modifier for the basicityand viscosity of the slag, or a dephosphorizing agent of molten steel.In addition, calcium hydroxide obtained by adding water to calcium oxideis used as a neutralizer for, for example, acids in a draining process.Therefore, recovering the Ca compounds contained in steelmaking slag,and reusing the same in the iron-making process is expected to reduceiron-making costs.

In the future, due to, for example, the change in the socialenvironment, there is a possibility that the number of publicengineering works utilizing steelmaking slag as, for example, road basematerials, construction materials and cement materials may decrease, orland that can be used for landfilling steelmaking slag may decrease.From the above view point, there is also a demand for reduction in thevolume of steelmaking slag that is reused or landfilled by recovering Cacompounds contained in the steelmaking slag.

Ca in steelmaking slag may be recovered by, for example, eluting thesame in an acidic aqueous solution, such as hydrochloric acid, nitricacid or sulfuric acid. However, the salts of the above compounds withthe acid generated in the method are difficult to reuse. For example,calcium chloride generated by eluting Ca from steelmaking slag intohydrochloric acid can be reused as an oxide by heating the same,however, the cost for processing toxic chloride gas generated during theheating is disadvantageously high. Recovering Ca in steelmaking slag byeluting the same into an acidic aqueous solution also require high costsfor purchasing acids and disposing the acids after the eluting process.

The disadvantage regarding the usage of acids, meanwhile, is expected tobecome irrelevant when Ca is recovered by eluting the same fromsteelmaking slag into an aqueous solution containing carbon dioxide(hereinafter, also simply referred to as a “CO₂ aqueous solution”) (seePTLs 1 to 3). Carbon dioxide is contained in an exhaust gas in a largeamount, and the desulfurization and denitration of the exhaust gasenable obtainment of a gas composed of substantially carbon dioxideother than air and water vapor. Industrially, a technology that extractscarbon dioxide from an exhaust gas is in practical use as described innon-patent literature (NPL) 4.

PTL 1 describes a method in which carbon dioxide is blown into anaqueous solution having therein calcium eluted from converter slag toallow calcium carbonate to settle out, followed by recovering the same.During this procedure, the lower limit of the pH is maintained at about10 for suppressing the generation of calcium hydrogen carbonate which ishighly soluble in water. Although PTL 1 does not describe a specific wayfor maintaining the pH at 10 or more, the inventors consider that the pHis maintained at 10 or more by adjusting the amount of blown carbondioxide.

PTL 2 describes a method in which fractured steelmaking slag isseparated into an iron-condensed phase and a phosphorus-condensed phase,calcium compositions in the phosphorus-condensed phase are dissolved inrinse water containing carbon dioxide dissolved therein, and then therinse water is heated to 50 to 60° C. to allow calcium hydrogencarbonate therein to settle out as calcium carbonate, followed byrecovering the same.

PTL 3 describes a method for recovering calcium compounds by eluting thecompounds from steelmaking slag in multiple processes. In the method,2CaO/SiO2 phase and phosphorus in a state of solid solution therein arepreferentially eluted by immersing steelmaking slag (pretreatment slag)several times in water containing carbon dioxide blown therein.

Steelmaking slag contains Fe, meanwhile, in the forms of an iron-basedoxide, calcium iron aluminum oxide and, in an extremely small amount,metal iron. The iron-based oxide contains Mn or Mg, and a small amountof elements, such as Ca, Al, Si, P, Ti, Cr and S. Calcium iron aluminumoxide also contains a small amount of elements, such as Si, P, Ti, Crand S. As used herein, the iron-based oxides include compounds formedfrom the iron-based oxides whose surface or the like is partly alteredto hydroxides or the like by, for example, water vapor in the air, andalso the calcium iron aluminum oxides include compounds formed from thecalcium iron aluminum oxides whose surface or the like is partly alteredto hydroxides, carbonates or the like by, for example, water vapor orcarbon dioxide in the air.

The iron-based oxides exist mostly as wustite (FeO), but also ashematite (Fe₂O₄) and magnetite (Fe₃O₄).

As the wustite and hematite contain magnetite (Fe₃O₄), which isferromagnetic, dispersed in the inside, they can be separated fromsteelmaking slag by magnetic separation. The magnetite that exists aloneor with other iron-based oxides can also be separated from steelmakingslag by magnetic separation.

PTLS 4 to 6 describe methods for modifying wustite to magnetite byoxidation or the like for separating larger amount of iron-based oxidesby magnetic separation.

The calcium iron aluminum oxide is magnetized to become magnetic, andthus can be separated from steelmaking slag by magnetic separation.

The iron-based oxides and calcium iron aluminum oxides contain only asmall amount of phosphorus (0.1 mass % or less), and thus can be used asa source for a blast furnace or sintering when separated fromsteelmaking slag by, for example, the magnetic separation and recovered(hereinafter, iron-based oxides and calcium iron aluminum oxides arealso collectively referred to as “iron-based compounds.” Calcium ironaluminum oxide is a Ca compound as well as an iron-based compound).

Metal iron is referred to Fe caught in slag during a steelmakingprocess, or minute Fe precipitated during the solidification ofsteelmaking slag. Larger metal iron is removed by magnetic separation orthe like during dry processes, such as fracturing or pulverizingsteelmaking slag in the air.

CITATION LIST Patent Literature

PTL 1

Japanese Patent Application Laid-Open No. S55-100220

PTL 2

Japanese Patent Application Laid-Open No. 2010-270378

PTL 3

Japanese Patent Application Laid-Open No. 2013-142046

PTL 4

Japanese Patent Application Laid-Open No. S54-88894

PTL 5

Japanese Patent Application Laid-Open No. S54-57529

PTL 6

Japanese Patent Application Laid-Open No. S52-125493

Non-Patent Literature

NPL 1

Masao NAKAGAWA “Current Status on the Effective Utilization of Iron andSteelmaking Slag” Lecture Text of Nos. 205 and 206 NISHIYAMA MemorialTechnical Course, The Iron and Steel Institute of Japan, June, 2011, p.25-56

NPL 2

“Kankyo-shizai Tecckou suragu (Environmental Materials, Iron and SteelSlag)” Nippon Slag Association, January, 2014

NPL 3

Takayuki FUTATSUKA et al., “Dissolution Behavior of Elements inSteelmaking Slag into Artificial Seawater” Tetsu-to-Hagane (Iron andSteel) Vol. 89, No. 4, January, 2014, P. 382-387

NPL 4

Masaki Iijima et al., “CO2 Capture Technology for Combating GlobalWarming and Climate Change” Mitsubishi Heavy Industries Technical ReviewVol. 47 No. 1, 2010, P. 47-53

SUMMARY OF INVENTION Technical Problem

As described above, recovery of Ca from steelmaking slag providesvarious advantages, and thus there is always a demand for increasing arecovery rate of Ca from steelmaking slag.

In the method described in PTL 1, a higher amount of blown carbondioxide leads to a pH lower than 10, and a lower amount of the blowncarbon dioxide leads to the decrease in the precipitation amount of Ca.Therefore, the amount of blown carbon dioxide should be preciselyadjusted for increasing a recovery rate of Ca, which makes a recoveryprocess complicated and increases recovery cost.

The method described in PTL 2 uses a mineral acid, and therefore a largeamount of the mineral acid and salt of calcium with the mineral acidremain after the precipitation and recovery of calcium as calciumcarbonate. Separation of these compounds requires a large amount ofwater and high-temperature heating. Accordingly, the method described inPTL 2 suffers complicated processes and increased recovery cost.Further, when steelmaking slag is washed with rinse water containingcarbon dioxide (aqueous solution containing calcium hydrogen carbonate)to dissolve calcium in the slag, the pH of the rinse water havingcalcium dissolved therein is neutral to weak alkaline as the rinse watercontains calcium hydrogen carbonate. When mixing the rinse water with aliquid leached by the mineral acid to neutralize the leached liquid andprecipitate calcium carbonate, the mixed liquid is acidified due to themineral acid, thereby increasing the dissolution amount (solubility) ofcalcium in the aqueous solution. This lowers the recovery efficiency ofcalcium because a large amount of calcium still remains in the mixedliquid even when calcium carbonate is precipitated.

In the method described in PTL 3, it is necessary to further increasethe number of processes of dissolving Ca compounds for increasing arecovery rate of Ca. This complicates a recovery process and a processfor uniting the recovered Ca compounds, and thus increases recoverycost.

The conventional methods thus have a disadvantage in that any effort forincreasing a recovery rate of Ca results in a complicated recoveryprocess to lengthen the recovery time, thereby increasing the recoverycost. The recovery rate of Ca would be easily increased when the amountof calcium compounds eluted into a CO₂ aqueous solution is increased.

However, PTLs 1 and 2 do not suggest any effort for increasing theamount of Ca compounds eluted into a CO₂ aqueous solution. The methoddescribed in PTL 3 may be capable of increasing total elution amount ofCa by increasing the number of processes of dissolving Ca compounds;however, as described above, this may results in the processes beingcomplicated and the recovery cost being increased in this method.

In view of the above disadvantage, the present invention is made with anobject to provide a method for eluting Ca from steelmaking slag, whichcan elute a larger amount of Ca from the steelmaking slag into a CO₂aqueous solution, and a method for recovering Ca eluted by the methodfor the eluting.

Solution to Problem

In view of the above object, the present invention relates to a methodfor eluting calcium from steelmaking slag, which includes a step ofremoving a compound containing iron from the steelmaking slag bysubjecting the steelmaking slag to magnetic separation; and a step ofcontacting the steelmaking slag subjected to the magnetic separationwith an aqueous solution containing carbon dioxide.

In addition, the present invention relates to a method for recoveringcalcium from steelmaking slag, which includes a step of eluting calciumfrom the steelmaking slag by the above method; and a step of recoveringthe eluted calcium.

Advantageous Effects of Invention

The present invention provides a method for eluting Ca from steelmakingslag, which can elute a larger amount of Ca from the steelmaking slaginto a CO₂ aqueous solution, and a method for recovering Ca eluted bythe method for the eluting.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart of a method according to the first embodiment ofthe present invention, for eluting Ca from steelmaking slag;

FIG. 2 is a flowchart illustrating a method according to the first modeof the second embodiment of the present invention, for eluting Ca fromsteelmaking slag;

FIG. 3 is a flowchart illustrating a method according to the second modeof the second embodiment of the present invention, for eluting Ca fromsteelmaking slag;

FIG. 4 is a flowchart illustrating a method according to the third modeof the second embodiment of the present invention, for eluting Ca fromsteelmaking slag;

FIG. 5 is a flowchart illustrating a method according to the fourth modeof the second embodiment of the present invention, for eluting Ca fromsteelmaking slag;

FIG. 6 is a flowchart of a method according to the third embodiment ofthe present invention, for recovering Ca eluted in a CO₂ aqueoussolution;

FIG. 7 is a flowchart illustrating an exemplary step of recovering Ca inthe third embodiment of the present invention; and

FIG. 8 is a graph showing the relationship between pH and the existenceratio of each carbonate species in a CO₂ aqueous solution.

DESCRIPTION OF EMBODIMENTS

Ca contained in steelmaking slag has a high solubility in a CO₂ aqueoussolution, while Fe has a low solubility therein. Therefore, according tothe present inventors' findings, when dissolving Ca in a CO₂ aqueoussolution, Fe remains on the surface of the steelmaking slag as ahydroxide, carbonate and/or hydrate thereof, or is precipitate on thesurface of the steelmaking slag after being eluted into the CO₂ aqueoussolution. The remaining or precipitated Fe on the steelmaking slagsurface may prevent the steelmaking slag surface from contacting the CO₂aqueous solution, thereby lowering the elution rate of Ca compared to anideal state.

During the procedure, Ca is expected to dissolve more easily in a CO₂aqueous solution by bringing the steelmaking slag into contact with theCO₂ aqueous solution while the steelmaking slag is pulverized or thesurface of the steelmaking slag is ground to make the contact of Ca inthe steelmaking slag to the CO₂ aqueous solution easier. However,Fe-containing compounds, such as iron-based compounds have high hardnessand also are less likely to be granulated by the reaction with the CO₂aqueous solution. Accordingly, large iron-based compounds remain as theyare, thereby making mechanical pulverizing or grounding of the Cacompounds difficult in some cases.

Further, calcium iron aluminum oxide containing Ca and Fe would havemore concentrated Al as Ca in the compound eluted. Al has a lowsolubility in a CO₂ aqueous solution as Fe. It is considered that theconcentrated Al, as Fe, also prevents the steelmaking slag surface fromcontacting the CO₂ aqueous solution, thereby further lowering theelution rate of Ca.

The present inventors have found that Ca compounds dissolve more easilyin a CO₂ aqueous solution by subjecting the steelmaking slag prior tobeing in contact with the CO₂ aqueous solution to magnetic separation,thereby removing the Fe-containing compounds, such as iron-basedcompounds, from the steelmaking slag, and completed the presentinvention. Subjecting steelmaking slag to magnetic separation prior tothe contacting with a CO₂ aqueous solution is considered to suppress theprevention of contact between the steelmaking slag surface and the CO₂aqueous solution due to the Fe remaining or precipitated on thesteelmaking slag surface, or suppress the prevention of pulverizing orgrounding due to the Fe-containing compounds, thereby allowing thecontact of Ca in the steelmaking slag to the CO₂ aqueous solutioneasier.

According to the present inventors' findings, calcium iron aluminumoxide becomes difficult to be magnetized after the elution of Catherefrom by the contact with the CO₂ aqueous solution, and thus therecovery of the calcium iron aluminum oxide by magnetic separationthereafter becomes difficult. Performing magnetic separation prior tothe contacting with the CO₂ aqueous solution, meanwhile, enablesrecovering calcium iron aluminum oxide in the steelmaking slag, as wellas reusing Fe derived from the calcium iron aluminum oxide.

In the following, more specific examples of the methods of the presentinvention for eluting Ca, and for recovering Ca will be described.

1. Method for Eluting Ca from Steelmaking Slag First Embodiment

FIG. 1 is a flowchart of a method according to the first embodiment ofthe present invention, for eluting Ca from steelmaking slag. Asillustrated in FIG. 1, the method according to the present embodimentfor eluting Ca from steelmaking slag includes a step of subjecting thesteelmaking slag to magnetic separation (Step S110: hereinafter alsoreferred to as “magnetic separation step”); and a step of bringing thesteelmaking slag subjected to the magnetic separation into contact witha CO₂ aqueous solution (Step S120: hereinafter also referred to as“contact step”).

Magnetic Separation Step: Magnetic Separation of Steelmaking Slag

In the present step, steelmaking slag is subjected to magneticseparation (Step S110).

Any steelmaking slag may be used as long as the steelmaking slag isdischarged during a steel making process. Examples of the steelmakingslag include converter slag, pretreatment slag, secondary refining slagand electric furnace slag.

Steelmaking slag discharged during a steel making process may be used asit is, or steelmaking slag fractured after the discharging may be used.When using fractured steelmaking slag, the fractured slag particlespreferably have a maximum particle diameter such that the size of theparticles becomes the same as or smaller than the structure of theiron-based compound, and preferably of 1,000 μm or less (hereinafter,the particles are also simply referred to as “slag particles,” andsimply referred “steelmaking slag” includes both the fractured particlesand non-fractured steelmaking slag). The maximum particle diameter of1,000 μm or less allows the iron-based compound to exist as a singleparticle, and thus the iron-based compound is more likely to beselectively captured. Slag particles with the maximum particle diameterof 1,000 μm or less have a larger surface area per volume, and allowwater or a CO₂ aqueous solution to satisfactorily permeate through thesteelmaking slag. This enables elution of a larger amount of Ca in abelow-described contact step. Any conventional crusher can fracturesteelmaking slag until the maximum particle diameter thereof fallswithin the above range.

From a similar view, the maximum particle diameter of the slag particlesis preferably 500 μm or less, more preferably 250 μm or less and evenmore preferably 100 μm or less. The maximum particle diameter of theslag particles can be reduced to fall within the above range by, forexample, further fracturing the fractured slag particles using acrusher, such as a hammer mill, a roller mill or a ball mill.

The steelmaking slag is preferably subjected to heat treatment beforethe magnetic separation. Subjecting the steelmaking slag to the heattreatment increases the magnetization of iron-based compound and metaliron, and thus a larger amount of the iron-based compound can be removedby the magnetic separation. The heat treatment is preferably performedat a temperature of 300° C. or more and 1,000° C. or less for 0.01minute or more and 60 minutes or less.

During the magnetic separation, steelmaking slag may be in a driedstate, but preferably is slurry dispersed in water. Slug particles inthe slurry-formed steelmaking slag are easily dispersed by, for example,the polarity of water molecules and/or a water flow, and thus theiron-based compound and metal iron are more likely to be selectivelycaptured by a magnetic force. In particular, slag particles having theparticle diameter of 1,000 μm or less are more likely to agglomerate ina gas, such as the air by the electrostatic force between the slagparticles, liquid bridge by agglomerated water vapor or the like, butare satisfactorily dispersed in slurry. Further, as the metal iron insteelmaking slag is minute, capturing the metal iron in driedsteelmaking slag is difficult, but metal iron dispersed in water of theslurry-formed steelmaking slag can be captured more easily by magneticseparation.

As the steelmaking slag, remaining-after-filtration slag may be used,which is obtained by placing steelmaking slag into a container includingwater therein, and leaching free lime, calcium hydroxide, and Ca on thetop surface of Ca compounds, followed by filtration. Using theremaining-after-filtration slag means using slag from which Ca has beenpartly eluted, and thus a load on a below-described contact step can bereduced.

Further, simultaneously obtained filtered water which contains leachedCa is a highly alkaline aqueous solution having a pH of 11 or more(hereinafter, also simply referred to as “slag-immersed water”). Theslag-immersed water can be used for precipitation of a solid componentcontaining Ca (precipitation step) during the recovery of Ca, asdescribed below. The slag-immersed water can also be used inapplications that require an alkaline aqueous solution, such as aneutralizer for acidic waste water. Further, subjecting theremaining-after-filtration slag to hydration treatment bybelow-described still standing with contained water gives advantagessuch that kneading with water is not necessary.

The magnetic separation may be performed by any conventional magneticseparator. The magnetic separator may be a wet type or a dry type, andcan be selected in accordance with the state of the steelmaking slag (ina dried state or a slurry form). In addition, the magnetic separator canbe appropriately selected from a drum type, a belt type, aflowing-between-fixed-magnets type and the like. The drum type magneticseparator is preferable as handling of the slurry-formed steelmakingslag is easy and the magnetic separation amount can be easily increasedby increasing the magnetic force in the drum type magnetic separator.The magnet used in the magnetic separator maybe a permanent magnet or anelectromagnet.

The magnetic flux density of a magnet may be at least at a level suchthat iron-based compounds and metal iron can be selectively capturedfrom other compounds contained in steelmaking slag, and, for example,0.003 T or more and 0.5 T or less, preferably 0.005 T or more and 0.3 Tor less, and more preferably 0.01 T or more and 0.15 T or less.

The magnetic separation is not necessarily performed until all theiron-based compounds contained in the steelmaking slag are removed. Evenif the magnetic separation removes only a small amount of iron-basedcompounds from steelmaking slag, the present invention can provide theeffect, namely easier elution of Ca into a CO₂ aqueous solution comparedto conventional methods. Accordingly, the period of time, number and thelike of the magnetic separation(s) may be appropriately selected inaccordance with, for example, effects of the magnetic separation on theproduction cost.

The steelmaking slag in a solid or slurry form after the removal ofiron-based compounds and metal iron by the magnetic separation may beused for a contact step as it is; however, steelmaking slag in a slurryform is preferably separated into steelmaking slag and a liquidcomponent by solid-liquid separation. The solid-liquid separation may beperformed by any conventional method, such as vacuum filtration orpressure filtration. The liquid component obtained by the solid-liquidseparation (hereinafter, also simply referred to as a“magnetic-separation water”) becomes alkaline as the magnetic-separationwater contains Ca eluted from the steelmaking slag in addition to waterused for forming the slurry. Therefore, the liquid component can be usedfor a below-described step of increasing the pH of a CO₂ aqueoussolution which has been in contact with steelmaking slag and thusincludes Ca eluted therein, during the precipitation of the eluted Ca.

The magnetic-separation-removed slag which is removed from steelmakingslag by the magnetic separation contains a large amount of Fe-containingcompounds, such as iron-based compounds and metal iron as describedabove, and thus can be reused as a source for a blast furnace orsintering.

Contact Step: Bringing Steelmaking Slag into Contact With CO₂ AqueousSolution

In the present step, steelmaking slag is brought into contact with a CO₂aqueous solution. Specifically, the steelmaking slag is immersed in theCO₂ aqueous solution to elute Ca contained in the steelmaking slag intothe aqueous solution (Step S120).

The steelmaking slag may be immersed in water in which carbon dioxide ispreviously dissolved, or the steelmaking slag may be immersed in waterfollowed by the dissolution of carbon dioxide in the water, in thepresent step. During the immersion of the steelmaking slag in the CO₂aqueous solution, it is preferred that the steelmaking slag and the CO₂aqueous solution are stirred for accelerating reactions.

Carbon dioxide can be dissolved in water by, for example, bubbling(blowing) a gas containing carbon dioxide. It is preferred that 30 mg/Lor more of non-ionized carbon dioxide (free carbonate) is dissolved inthe CO₂ aqueous solution for increasing elution of Ca from thesteelmaking slag. The amount of free carbonate that may be contained intap water in general is 3 mg/L or more and 20 mg/L or less.

The gas containing carbon dioxide may be pure carbon dioxide gas, or agas containing carbon dioxide and components, such as oxygen andnitrogen in addition. Examples of the gases containing carbon dioxideinclude an exhaust gas after combustion, and a mixed gas of carbondioxide, air and water vapor. For increasing elution of Ca compounds(e.g., calcium silicate) from the steelmaking slag into a CO₂ aqueoussolution by increasing the concentration of carbon dioxide in the CO₂aqueous solution, the gas containing carbon dioxide preferably containscarbon dioxide in high concentration (e.g. 90%).

During the elution of Ca, the amount of carbon dioxide in the CO₂aqueous solution decreases with the dissolution of Ca since Ca reactswith carbon dioxide and forms water-soluble calcium hydrogen carbonate.Therefore, it is preferred to keep providing carbon dioxide to the CO₂aqueous solution after the steelmaking slag is immersed in the CO₂aqueous solution.

For satisfactorily eluting Ca contained in the steelmaking slag, theamount of the steelmaking slag in a CO₂ aqueous solution is preferably 1g/L or more or 100 g/L or less, and more preferably 2 g/L or more or 40g/L or less. The immersion is performed preferably for three minutes ormore, and more preferably for five minutes or more, for satisfactorilyeluting Ca contained in the steelmaking slag.

Effects

The first embodiment is capable of easier elution of Ca compoundscontained in steelmaking slag into a CO₂ aqueous solution, and thus alarger amount of Ca can be eluted into the CO₂ aqueous solution in ashorter period of time. In addition, the first embodiment can beperformed easily, and thus the costs during the actual performance ofthe embodiment can be reduced.

Modification of First Embodiment

In the first embodiment, the contact step may be a step (hereinafter,also referred to as a “modified contact step”) of bringing steelmakingslag subjected to the magnetic separation into contact with an aqueoussolution containing carbon dioxide while fracturing or pulverizing thesteelmaking slag, or grinding the surface of the steelmaking slag(hereinafter, also collectively and simply referred to as a “fracturingor the like”).

Modified Contact Step: Bringing Steelmaking Slag into Contact With CO₂Aqueous Solution While Fracturing or the Like

The elution of Ca from steelmaking slag occurs upon contact of Cacompounds or Ca with a CO₂ aqueous solution in the vicinity of thesurface of the steelmaking slag or inside the steelmaking slag.Therefore, during the contacting of steelmaking slag with a CO₂ aqueoussolution, subjecting the steelmaking slag in contact with the CO₂aqueous solution to fracturing or the like increases the surface area ofslag particles that can be in contact with the CO₂ aqueous solution, andthus it becomes possible to bring a larger amount of Ca into contactwith the CO₂ aqueous solution, or to increase the amount of the CO₂aqueous solution permeating through the inside of the slag particles.

In addition, when components contained in the steelmaking slag dissolvein the CO₂ aqueous solution, for example, Fe, Al, Si and/or Mn, and/orhydroxides, carbonates and/or hydrates thereof may remain or beprecipitated on the surface of the steelmaking slag. When the remainingor precipitated substances prevent the steelmaking slag from contactingthe CO₂ aqueous solution, Ca becomes less likely to be eluted from theinside of the steelmaking slag. Subjecting the steelmaking slag incontact with the CO₂ aqueous solution to fracturing or the likecontinuously forms new surfaces where the above described substances donot remain or are not precipitated, and the CO₂ aqueous solution canpermeate through the steelmaking slag from the newly formed surfaces,and therefore, Ca in the inside of the steelmaking slag also becomesmore likely to be eluted. In addition, grinding the surface of thesteelmaking slag removes the remaining or precipitated substances toincrease the contact area between the CO₂ aqueous solution and the slagparticles and allow the CO₂ aqueous solution to permeate more easilythrough the steelmaking slag.

Specifically, steelmaking slag is immersed in a CO₂ aqueous solution,and at the same time, a conventional crusher usable in a wet method isused, thereby fracturing the immersed steelmaking slag. Alternatively,slag particles may be subjected to pulverizing and at the same time tofracturing or the like in a CO₂ aqueous solution by rotating a ball millcharged with the slag particles, the CO₂ aqueous solution andpulverizing balls.

The present step is preferably continued until the maximum particlediameter of the slag particles becomes 1,000 μm or less, preferably 500μm or less, more preferably 250 μm or less, and even more preferably 100μm or less, for increasing the elution amounts of calcium.

Effects

The modification of the first embodiment is capable of eluting a largeramount of Ca into the CO₂ aqueous solution in an even shorter period oftime. In addition, the present modification can be performed easily, andthus the costs during the actual performance of the modification can bereduced.

Second Embodiment

Each of FIGS. 2 to 5 is a flowchart of a method according to anotherembodiment of the present invention, for eluting Ca from steelmakingslag. As illustrated in FIGS. 2 to 5, each method according to thepresent embodiment for eluting Ca from steelmaking slag includes a stepof subjecting steelmaking slag to magnetic separation (Step S110:magnetic separation step); at least one of steps of subjecting thesteelmaking slag to hydration treatment (Steps S130-1 to S130-5:hereinafter also referred to as “hydration step” each); and bringing thesteelmaking slag subjected to the magnetic separation and hydrationtreatment into contact with a CO₂ aqueous solution (Step S120: contactstep). The description for the magnetic separation step and contact stepis omitted in the present embodiment as these steps can be performed inthe same manner as in the above described first embodiment.

Hydration Step: Hydration Treatment of Steelmaking Slag

The present step may be performed after the magnetic separation step asillustrated in FIG. 2, before the magnetic separation step asillustrated in FIG. 3, before and after the magnetic separation step asillustrated in FIG. 4, or simultaneously with the magnetic separationstep as illustrated in FIG. 5. Further, the hydration step may beperformed before and simultaneously with the magnetic separation step,simultaneously with and after the magnetic separation step, or before,simultaneously with and after the magnetic separation step. Thehydration step may be repeated more than once, for example, before,after and/or simultaneously with the magnetic separation step.

As described above, steelmaking slag contains Ca in the forms of, forexample, free lime, calcium hydroxide (Ca(OH)₂), calcium carbonate(CaCO₃), calcium silicate (such as Ca₂SiO₄ or Ca₃SiO₅) and calcium ironaluminum oxide (Ca₂(Al_(1-X)Fe_(X))₂O₅).

When subjecting the steelmaking slag to the hydration treatment,obtained are, for example, calcium silicate hydrates and calciumhydroxide (Ca(OH)₂) from calcium silicate by the reaction represented bythe following equation 1, and hydroxides of calcium oxides from calciumiron aluminum oxide by the reaction represented by the followingequation 2 (hereinafter, compounds containing calcium, which can beobtained by the hydration treatment are also collectively referred to as“Ca hydrates”):

2(2CaO.SiO₂)+4H₂O→3CaO.2SiO₂.3H₂O+Ca(OH)₂   (Equation 1)

2CaO.½(Al₂O₃.Fe₂O₃)+10H₂O→½(4CaO.Al₂O₃.19H₂O)+HFeO₂   (Equation 2)

(equation 2 is an example in which X is ½ in calcium iron aluminum oxide(Ca₂(Al_(1-X)Fe_(X))₂O₅)).

The Ca hydrates generated, for example, by the above reactions easilydissolve in a CO₂ aqueous solution. Subjecting steelmaking slag tohydration treatment thus makes it possible to elute Ca derived from, forexample, calcium silicate and calcium iron aluminum oxide contained inthe steelmaking slag more easily.

The free lime, which is easily dissolved in a CO₂ aqueous solution,typically exists in steelmaking slag only in an amount of less thanabout 10 mass %. On the other hand, calcium silicate and calcium ironaluminum oxide typically exists in steelmaking slag in an amount ofabout 25 mass % to 70 mass %, and about 2 mass % to 30 mass %,respectively. Therefore, when the hydration treatment can elute Cacontained in, for example, calcium silicate and calcium iron aluminumoxide into a CO₂ aqueous solution more easily, the amount of Ca elutedfrom steelmaking slag into the CO₂ aqueous solution may increase, and itis possible to shorten the time for recovering Ca from the steelmakingslag.

In addition, the total volume of compounds generated by the hydrationtreatment is usually larger than the total volume of the correspondingcompounds before the reaction. During the hydration treatment, free limein the steelmaking slag is partly eluted into water used for thetreatment. Subjecting steelmaking slag to hydration treatment thusgenerates cracks inside slag particles, and the slag particles are morelikely to disintegrate from these cracks. Such disintegration of theslag particles reduces the particle diameter of the slag particles, andincreases the surface area per volume thereof, as well as allowing wateror a CO₂ aqueous solution to satisfactorily permeate through thesteelmaking slag. This enables hydration of a large amount of Cacompounds in the present step, and elution of a larger amount of Ca inthe contact step.

Hydration treatment may be performed by a method and under conditionswhich can hydrate any Ca compound contained in steelmaking slag,preferably calcium silicate or calcium iron aluminum oxide.

Specific examples of the hydration treatment include, for example, thefollowing methods: leaving steelmaking slag which is immersed and sunkin water to stand (hereinafter, also simply referred to as a “immersionand still standing”); stirring steelmaking slag immersed in water(hereinafter, also simply referred to as a “immersion and stirring”);leaving a paste containing slag particles and water to stand(hereinafter, also simply referred to as a “still standing in pasteform”); and leaving steelmaking slag to stand in a container including asatisfactory amount of water vapor (hereinafter, also simply referred toas a “still standing in wet condition”). Any one of these methods cansatisfactorily contact steelmaking slag with water. The hydrationtreatment may be performed by only one of the methods, namely theimmersion and still standing, the immersion and stirring, the stillstanding in paste form, the still standing in wet condition and thelike, or by two or more of the above methods in any order.

When performing the hydration treatment by the still standing in wetcondition, the relative humidity is preferably 70% or more for ensuringsatisfactory condensing water vapor between slag particles by capillarycondensation to allow water reach the slag particles.

When simultaneously performing the magnetic separation and hydrationtreatment, the slurry-formed steelmaking slag may be in contact withwater for a long period of time by, for example, performing the magneticseparation in a wet method for a long period of time, or circulating theslurry when subjecting to the magnetic separation in a wet method. Forcirculating the slurry, the slurry may be immersed and stirred in a tankprovided on a circulation path.

During the performance of the hydration treatment by the immersion andstill standing, the immersion and stirring, or the still standing inpaste form, water used therefor preferably contains less than 300 mg/lof carbon dioxide including, for example, non-ionized free carbonate,and ionized hydrogen carbonate ions (HCO₃ ⁻). With the carbon dioxidecontent being less than 300 mg/l, Ca compounds other than free lime andcalcium hydroxide are less likely to be eluted into the water used forthe hydration treatment, and thus most of Ca contained in steelmakingslag can be eluted into the CO₂ aqueous solution in the contact step,which less likely complicates a Ca recovery process. Further, when thewater contains a large amount of carbon dioxide, carbon dioxide isreacted with Ca eluted from free lime, calcium hydroxide and the like,and generated calcium carbonate is precipitated to cover the surface ofthe slag particles, which makes hydration reactions less likely toproceed. However, when the carbon dioxide content is less than 300 mg/l,the prevention of hydration reactions caused by the precipitation ofcalcium carbonate is less likely to occur. Industrial water usuallycontains less than 300 mg/l of carbon dioxide, and therefore, industrialwater to which carbon dioxide is not intentionally added or mixed ispreferred as the water to be used for the hydration treatment performedby the immersion and still standing, and the immersion and stirring.

During the performance of the hydration treatment by the immersion andstill standing, the immersion and stirring, or the still standing inpaste form, the water used for the hydration treatment may have atemperature at which the water is not violently evaporated at the most.For example, the temperature of the water is preferably 100° C. or lesswhen steelmaking slag is subjected to the hydration treatment atsubstantially atmospheric pressure. When the hydration treatment isperformed under a higher pressure by using, for example, an autoclave,the water temperature may be more than 100° C., as long as thetemperature is lower than the boiling point of the water under thepressure applied during the hydration treatment. Specifically, the watertemperature during the hydration treatment performed by the immersionand still standing, or the immersion and stirring is preferably 0° C. ormore and 80° C. or less. While there is no limitation to the upper limitof the water temperature when performing the hydration treatment under ahigher pressure by using, for example, an autoclave, a temperature of300° C. or less is preferred from the view point of the pressureresistance property of an apparatus, and in an economical aspect. Thetemperature during the performance of the hydration treatment by thestill standing in paste form is preferably 0° C. or more and 70° C. orless.

When subjecting steelmaking slag to the hydration treatment by the stillstanding in wet condition, the relative humidity may be increased byintroducing water vapor into a gas, such as air, nitrogen (N₂), oxygen(O₂), argon (Ar) or helium (He), or a gas composed only of water vapormay be used. While the relative humidity and temperature in thecontainer may be set to any values, the temperature of a gas at whichwater vapor is introduced under, for example, substantially atmosphericpressure may be 0° C. or more and 100° C. or less, preferably 10° C. ormore and 100° C. or less, and the relative humidity of the gas may be70% or more. The gas may be stirred for more uniform hydration of Cacompounds.

When subjecting steelmaking slag to the hydration treatment by the stillstanding in wet condition using a gas composed only of water vapor, itis preferred to heat the water vapor, thereby increasing the water vaporpressure to the atmospheric pressure or more. Setting the water vaporpressure to atmospheric pressure or more enables easier filling of thecontainer with water vapor, which can, for example, omit pressurereduction previous to the introduction of water vapor into thecontainer. This enables reduction of facility cost and management cost,thereby performing easier and less expensive hydration treatment. Whilethe water vapor pressure is set to the atmospheric pressure or more, thewater vapor temperature may be, for example, 100° C. or more. As withthe case of an autoclave, while there is no limitation to the upperlimit of the water temperature, a temperature of 300° C. or less ispreferred from the view point of the pressure resistance property of anapparatus, and in an economical aspect.

The duration of the hydration treatment may be set to any period of timein accordance with, for example, the average particle size of the slagand the temperature during the hydration treatment (the temperature ofwater or air containing water vapor). A smaller average particle size ofthe slag or a higher temperature during the hydration treatment enablesperformance of the hydration treatment in a shorter duration.

For example, when subjecting steelmaking slag, with the maximum particlediameter of the slag particles thereof of 1,000 μm or less, to thehydration treatment by the immersion and still standing or the immersionand stirring at normal temperature, the duration of the hydrationtreatment may be about 8 hours without interval, preferably 3 hours ormore and 30 hours or less. When performing the hydration treatment byimmersion in water at 40° C. or more and 70° C. or less, the duration ofthe hydration treatment is preferably 0.6 hours or more and 8 hours orless, without interval.

In addition, when subjecting steelmaking slag, with the maximum particlediameter of the slag particles thereof of 1,000 μm or less, to thehydration treatment by the still standing in paste form at normaltemperature, the duration of the hydration treatment may be about 7hours without interval, preferably 3 hours or more and 30 hours or less.When performing the hydration treatment by the still standing withcontained water at 40° C. or more and 60° C. or less, the duration ofthe hydration treatment is preferably 0.5 hours or more and 8 hours orless without interval.

When subjecting steelmaking slag, with the maximum particle diameter ofthe slag particles thereof of 1,000 μm or less, to the hydrationtreatment by the still standing in wet condition under the atmospherewith relative humidity of 90% and at normal temperature, the duration ofthe hydration treatment may be about 10 hours without interval,preferably 1 hour or more and 40 hours or less. During the performanceof the hydration treatment using a gas composed only of water vapor at100° C. or more, the duration of the hydration treatment is preferably0.2 hours or more and 5 hours or less without interval.

Predetermined average particle size of steelmaking slag and conditionsfor the hydration treatment (e.g., temperature and duration) forsatisfactorily increasing the Ca recovery rate (for example, no higherrecovery rate can be obtained) can be determined, and referred to fromthe next hydration treatment.

In addition, the hydration treatment is preferably continued untilcalcium silicate satisfactorily becomes hydrates and calcium hydroxide,and/or calcium iron aluminum oxide satisfactorily becomes hydroxides ofcalcium oxides. The hydration treatment is, for example, preferablycontinued until the amount of calcium silicate or calcium iron aluminumoxide contained in steelmaking slag becomes 50 mass % or less, or 20mass % or less, respectively.

The steelmaking slag in a solid or slurry form after subjected to thehydration treatment may be used for the contact step as it is; however,when the hydration treatment is performed by the immersion and stillstanding, the immersion and stirring, or the still standing in pasteform, the steelmaking slag subjected to the hydration treatment ispreferably separated into steelmaking slag and a liquid component bysolid-liquid separation. The solid-liquid separation may be performed byany conventional method, such as vacuum filtration or pressurefiltration. The liquid component obtained by the solid-liquid separation(hereinafter, also simply referred to as a “hydration-treatment water”)becomes alkaline as the hydration-treatment water contains Ca elutedfrom the steelmaking slag. Therefore, the liquid component can be usedfor a below-described step of increasing the pH of a CO₂ aqueoussolution, which has been in contact with steelmaking slag and thusincludes Ca eluted therein, during the precipitation of the eluted Ca.

First Mode of Second Embodiment

FIG. 2 is a flowchart illustrating a method according to the first modeof the present embodiment, for eluting Ca from steelmaking slag. Asillustrated in FIG. 2, the magnetic separation step (Step S110), thehydration step (Step S130-1) and the contact step (Step S120) areperformed in this order in the present mode.

The hydration treatment performed on steelmaking slag in the hydrationstep (Step S130-1) may be any one of the immersion and still standing,the immersion and stirring, the still standing in paste form and thestill standing in wet condition; however, for simplifying subsequentsteps by simultaneously performing the hydration treatment and thesolid-liquid separation, the immersion and still standing or the stillstanding in paste form is preferred.

The magnetic separation reduces the volume of steelmaking slag, andtherefore, performing the hydration step (Step S130-1) after themagnetic separation step (Step S110) can subject the steelmaking slag tothe hydration treatment more effectively.

Second Mode of Second Embodiment

FIG. 3 is a flowchart illustrating a method according to the second modeof the present embodiment, for eluting Ca from steelmaking slag. Asillustrated in FIG. 3, the hydration step (Step S130-2), the magneticseparation step (Step S110), and the contact step (Step S120) areperformed in this order in the present mode.

The hydration treatment performed on steelmaking slag in the hydrationstep (Step S130-2) may be any one of the immersion and still standing,the immersion and stirring, the still standing in paste form and thestill standing in wet condition; however, for dispersing agglomeratedslurry particles, thereby more selectively capturing iron-basedcompounds and metal iron in the following magnetic separation step (StepS110), the immersion and stirring is preferred.

Third Mode of Second Embodiment

FIG. 4 is a flowchart illustrating a method according to the third modeof the present embodiment, for eluting Ca from steelmaking slag. Asillustrated in FIG. 4, the hydration step (Step S130-3), the magneticseparation step (Step S110), the hydration step (Step S130-4), and thecontact step (Step S120) are performed in this order in the presentmode.

The hydration treatment performed on steelmaking slag in the hydrationsteps (Step S130-3 and Step S130-4) may be any one of the immersion andstill standing, the immersion and stirring, the still standing in pasteform and the still standing in wet condition. As in the second mode, thehydration treatment performed on steelmaking slag in the hydration step(Step S130-3) before the magnetic separation is preferably the immersionand stirring for dispersing agglomerated slurry particles, thereby moreselectively capturing iron-based compounds and metal iron in thefollowing magnetic separation step (Step S110). As in the second mode,the hydration treatment performed on steelmaking slag in the hydrationstep (Step S130-4) after the magnetic separation is preferably theimmersion and still standing or the still standing in paste form forsimplifying subsequent steps by simultaneously performing the hydrationtreatment and the solid-liquid separation.

Fourth Mode of Second Embodiment

FIG. 5 is a flowchart illustrating a method according to the fourth modeof the present embodiment, for eluting Ca from steelmaking slag. Asillustrated in FIG. 5, the hydration step and the magnetic separationstep are simultaneously performed (Step S110/S130-5), followed by thecontact step (Step S120) in the present mode.

The hydration step and the magnetic separation step can besimultaneously performed by subjecting slurry-formed steelmaking slag tothe magnetic separation for the period of time sufficient forsatisfactorily hydrating Ca compounds contained in the steelmaking slag,particularly calcium silicate that is difficult to capture by themagnetic separation. For example, about one hour of magnetic separationon slurry-formed steelmaking slag can simultaneously perform thehydration treatment and the magnetic separation.

In addition, the immersion and still standing or the immersion andstirring may be performed in a storage tank by circulating the slurrybetween the magnetic separator and the storage tank at a circulationrate adjusted so that the flow rate of the slurry in the tank becomessatisfactorily slow.

Effects

The second embodiment is capable of hydrating Ca compounds contained insteelmaking slag, particularly calcium silicate and calcium ironaluminum oxide, to generate Ca hydrates which can be eluted into a CO₂aqueous solution more easily, and thus a larger amount of Ca can beeluted into the CO₂ aqueous solution in a shorter period of time. Inaddition, the hydration treatment in the second embodiment can beperformed easily, and thus the costs during the actual performance ofthe embodiment can be reduced.

First Modification of Second Embodiment

When the hydration step is the immersion and stirring in the first tothird modes of the second embodiment, the immersed steelmaking slag maybe subjects to the fracturing or the like at the same time (hereinafteralso referred to as “modified hydration step”).

Modified Hydration Step: Bringing Steelmaking Slag into Contact With CO₂Aqueous Solution While Fracturing or the Like

Reactions during the above described hydration treatment occur uponcontact of Ca compounds with water in the vicinity of the surface of thesteelmaking slag or inside the steelmaking slag. While water permeatesthrough the steelmaking slag to some extent, a larger amount of watercontacts the vicinity of the surface. Ca hydrates is thus more likely tobe generated in the vicinity of the surface of the steelmaking slag. Inaddition, when components contained in the steelmaking slag dissolve inthe water used for the hydration treatment, for example, Fe, Al, Si andMn, and/or hydroxides, carbonates and/or hydrates thereof may remain orbe precipitated on the surface of the steelmaking slag, as in the abovedescribed case of dissolution into a CO₂ aqueous solution. When theremaining or precipitated substances prevent water from permeatingthrough the steelmaking slag, Ca hydrates becomes less likely to begenerated inside the steelmaking slag.

Subjecting the steelmaking slag immersed in water to fracturing or thelike during the hydration treatment, meanwhile, increases the surfacearea of slag particles, thereby increasing the contact area between thewater and the slag particles. Subjecting the steelmaking slag immersedin water to fracturing or the like also continuously forms new surfaceswhere the above described substances do not remain or are notprecipitated, and water can permeate through the steelmaking slag fromthe continuously formed new surfaces, and therefore, Ca hydrates becomesmore likely to be generated even inside the steelmaking slag. Inaddition, grinding the surface of the steelmaking slag removes theremaining or precipitated substances to increase the contact areabetween the water and the slag particles and allow water to permeatemore easily through the steelmaking slag.

Specifically, steelmaking slag is immersed in water, and at the sametime, a conventional crusher usable in a wet method is used, therebyfracturing the immersed steelmaking slag. Alternatively, performing thehydration treatment and pulverizing slag particles in water at the sametime by rotating a ball mill charged with the slag particles, water andpulverizing balls.

The present step can easily elute Ca more quickly in an amount the sameor more as compared to the above embodiment including the first step.For example, when using steelmaking slag with the maximum particlediameter thereof of 1,000 μm or less, the duration of the present stepis preferably 0.1 hour or more and 5 hours or less, and more preferably0.2 hours or more and 3 hours or less, without interval.

The present step is preferably continued until the maximum particlediameter of the slag particles becomes 1,000 μm or less, preferably 500μm or less, more preferably 250 μm or less, and even more preferably 100μm or less. Therefore, the present step can be performed simultaneouslywith the fracturing steelmaking slag or pulverizing slag particleswithout complicating the steps.

In particular, when the hydration step (Step S130-2 or Step S130-3) isperformed before the magnetic separation step (Step S110) as in thesecond or third mode of the second embodiment, performing the modifiedhydration step in place of the hydration step can reduce the particlediameter of steelmaking slag to fall within the above range whileperforming the hydration treatment, and thus the load on the fracturingor the like of the steelmaking slag can also be reduced.

Effects

The first modification is capable of eluting a larger amount of Ca intoa CO₂ aqueous solution in an even shorter period of time. In addition,the present modification can be performed without complicating thesteps, such as being performed simultaneously with the fracturingsteelmaking slag or the pulverizing slag particles, and thus the costsduring the actual performance of the embodiment can be reduced.

Second Modification of Second Embodiment

In the second embodiment, the contact step may also be a step (modifiedcontact step) of bringing steelmaking slag subjected to the magneticseparation into contact with an aqueous solution containing carbondioxide while subjecting the steelmaking slag to the fracturing or thelike.

The second modification of the second embodiment is capable of eluting alarger amount of Ca into the CO₂ aqueous solution in an even shorterperiod of time. The present modification can be performed easily, andthus the costs during the actual performance of the modification can bereduced.

2. Method for Recovering Ca from Steelmaking Slag

FIG. 6 is a flowchart of a method according to the third embodiment ofthe present invention for recovering Ca eluted in a CO₂ aqueoussolution. As illustrated in FIG. 6, the present embodiment includes astep of eluting Ca (Step S100) and a step of recovering Ca (Step S200).

Step of Eluting Ca

In the step of eluting Ca (Step S100), Ca is eluted from steelmakingslag. Any one of the above described first and second embodiments andmodifications thereof is capable of eluting Ca.

Step of Recovering Ca

FIG. 7 is a flowchart illustrating an exemplary step of recovering Ca(Step S200). As illustrated in FIG. 7, the step of recovering Ca (StepS200) includes for its performance, for example, a step of separatingsteelmaking slag from a CO₂ aqueous solution (Step S210 hereinafter alsoreferred to as “separation step”), a step of precipitating Ca (StepS220: hereinafter also referred to as “precipitation step”), and a stepof recovering a precipitated solid component (Step S230: hereinafteralso referred to as “recovery step”).

Separation Step: Separation of Steelmaking Slag from CO₂ AqueousSolution

In the present step, steelmaking slag is separated from a CO₂ aqueoussolution (supernatant) having Ca dissolved therein (Step S210). Theseparation may be performed by any conventional method. Examples ofseparating methods include filtration, and a method in which thesteelmaking slag settles out by leaving the CO₂ aqueous solution tostand. In the case where the slag settles out, only the supernatant mayfurther be recovered, or subsequent steps may be performed only on thesupernatant in a two-component system containing the supernatant and thesteelmaking slag that settles out, as long as a solid componentprecipitated in a subsequent step is not mixed with the steelmakingslag.

The present step becomes unnecessary when the steelmaking slag hasalready been separated from the CO₂ aqueous solution in the contactstep.

Precipitation Step: Precipitation of Solid Component Containing Ca

In the present step, Ca eluted into the CO₂ aqueous solution isprecipitated as a solid component containing Ca (Step S220). Ca elutedinto the CO₂ aqueous solution can be precipitated by any conventionalmethod. Examples of the methods for precipitating Ca eluted in a CO₂aqueous solution as a solid component include a method that removescarbon dioxide from the CO₂ aqueous solution, and a method thatincreases the pH of the CO₂ aqueous solution.

Removal of Carbon Dioxide

Ca eluted into a CO₂ aqueous solution in the contact step (Step S120)can be precipitated by, for example, removing carbon dioxide from theCO₂ aqueous solution separated from steelmaking slag in the separationstep (Step S210). Examples of Ca compounds to be precipitated in thisprocedure include calcium carbonate, calcium carbonate hydrate andcalcium hydroxide.

Any method can be employed for removing carbon dioxide from a CO₂aqueous solution. Examples of the methods for removing carbon dioxideinclude (1) gas introduction, (2) pressure reduction and (3) heating.

(1) Gas Introduction

Carbon dioxide can be removed from a CO₂ aqueous solution having Cadissolved therein by introducing into the CO₂ aqueous solution a gashaving a partial pressure of carbon dioxide lower than the equilibriumpressure of carbon dioxide dissolved in the CO₂ aqueous solution toreplace the dissolved carbon dioxide by the introduced gas, or todiffuse (transfer) carbon dioxide into bubbles of the introduced gas.While the gas to be introduced may be a gas reactive to water (such aschlorine and sulfur dioxide), a gas having low reactivity to water ispreferred for suppressing the reduction of the precipitated Ca amountdue to the formation of salts from the eluted calcium and ions generatedby the introduction of a gas reactive to water into the CO₂ aqueoussolution. The gas to be introduced into the CO₂ aqueous solution may bean inorganic gas or an organic gas. An inorganic gas is more preferredsince the possibility of combustion or explosion is low when the gasleaks outside. Examples of the inorganic gases having low reactivity towater include air, nitrogen, oxygen, hydrogen, argon, helium and mixedgases thereof. An example of the mixed gas is air in the environmentwhere the present step is performed, which contains nitrogen and oxygenin an approximate ratio of 4 to 1. Examples of the organic gases havinglow reactivity to water include methane, ethane, ethylene, acetylene,propane and fluorocarbons.

(2) Pressure Reduction

Under the pressure environment of about one atmospheric pressure (about100 kPa) or less, the solubility of carbon dioxide decreases when thepressure applied on a CO₂ aqueous solution decreases. Therefore, carbondioxide can be removed from the CO₂ aqueous solution by placing the CO₂aqueous solution under a reduced-pressure environment. For example,carbon dioxide can be removed by putting the CO₂ aqueous solution intoan airtight container and evacuating air (degassing) of the containerusing, e.g., a pump to allow the container to have a reduced-pressureatmosphere. For further increasing the amount of removed carbon dioxide,the pressure reduction may be simultaneously performed with applyingultrasonic waves to the CO₂ aqueous solution or stirring the CO₂ aqueoussolution.

(3) Heating

Under the pressure environment of about one atmospheric pressure (about100 kPa) or less, the solubility of carbon dioxide decreases when thetemperature of a CO₂ aqueous solution increases. Therefore, carbondioxide can be removed from the CO₂ aqueous solution by heating the CO₂aqueous solution. For lowering heating costs, the CO₂ aqueous solutionis preferably heated to a temperature within such a range that the vaporpressure of the solution does not exceed the pressure in the atmosphere.When the pressure in the atmosphere is one atmospheric pressure, forexample, the heating temperature is preferably less than 100° C. Heatingthe CO₂ aqueous solution not only removes carbon dioxide, but alsolowers the solubility of Ca compounds (e.g., calcium carbonate), andthus Ca can be precipitated more easily.

The above methods (1) to (3) may be performed in a combination forfurther increasing the amount of removed carbon dioxide. The mostsuitable combination can be selected in view of, for example, a deliverysystem of a gas or heat, a site location, or availability of aby-product gas in a factory.

For example, while keeping introducing a gas into the CO₂ aqueoussolution, air evacuation is performed in an amount that is the same asor more than the amount of the introduced gas to achieve areduced-pressure atmosphere. In such a manner, the gas introduction canprovide effects of removing carbon dioxide and stirring, and thepressure reduction of the CO₂ aqueous solution can provide an effect ofremoving carbon dioxide, and thus carbon dioxide can be removedeffectively. Heating in addition to this procedure can furtheraccelerate removal effects of carbon dioxide. Since carbon dioxide canbe easily removed due to the additive effects of the gas introductioninto the CO₂ aqueous solution and pressure reduction of the CO₂ aqueoussolution, the heating temperature is not necessarily high, and thusheating costs can be reduced.

Increasing pH

Increasing the pH of the CO₂ aqueous solution separated from thesteelmaking slag can precipitate a solid component containing calcium inthe CO₂ aqueous solution. The existence state of carbon dioxide in anaqueous solution can be represented by the equations 3 to 5 below. Theequilibrium is achieved as shown in equations 3 to 4 until about pH 8.5,and in equations 4 to 5 at pH 8.5 and above. FIG. 8 shows the existenceratio of each of ions and substances. In FIG. 8, [H₂CO₃*] represents thetotal amount of [CO₂] and [H₂CO₃]. It can be considered from FIG. 8 thatcalcium is precipitated by generation of poorly-soluble calciumcarbonate (CaCO₃) from calcium ion (Ca²⁺) bonding to hydrogen carbonateion (HCO₃ ⁻) until about pH 8.5 as represented in equation 6. At pH 8.5and above, calcium is considered to be precipitated by generation ofpoorly-soluble calcium carbonate (CaCO₃) from a bonding reaction ofcalcium ion and carbonate ion as represented in equation 7. FIG. 8showing the relationship between the existence state of carbon dioxideand the pH is described in known references (e.g., “Fushoku⋅BoushokuHandobukku (Corrosion-Anticorrosion handbook),” page 155, published in2000, by Japan Society of Corrosion Engineering).

CO₂+H₂⇄H₂CO₃   (Equation 3)

H₂CO₃⇄H⁺+HCO₃ ⁻  (Equation 4)

HCO₃ ⁻⇄H⁺+CO₃ ²⁻  (Equation 5)

Ca²⁺+HCO₃ ⁻⇄H⁺+CaCO₃   (Equation 6)

Ca²⁺+CO₃ ²⁻⇄CaCO₃   (Equation 7)

Upon increasing the pH of an aqueous solution that is obtained bybringing steelmaking slag into contact with an aqueous solutioncontaining carbon dioxide to elute calcium, followed by solid-liquidseparation, substantially all of Ca is precipitated until the pH becomes8.5, and therefore the reaction between the calcium ion and the hydrogencarbonate ion is most common.

When the precipitation of Ca starts, cloudiness caused by calciumcarbonate is generated in the CO₂ aqueous solution. It is sufficient tocontinue to increase the pH of the CO₂ aqueous solution until thecloudiness can be confirmed by visual observation. For furtherincreasing the recovery rate of Ca by precipitating Ca moresufficiently, the pH of the CO₂ aqueous solution separated from thesteelmaking slag in the separation step (Step S210) is increasedpreferably by 0.2 or more, more preferably by 0.3 or more, still morepreferably by 1.0 or more, even more preferably by 1.5 or more, andparticularly preferably by 2.0 or more.

The pH of the CO₂ aqueous solution is preferably increased whilemeasuring the same. The pH of the CO₂ aqueous solution can be measuredby any conventional glass electrode method.

While solid components containing not only Ca but also other elements,such as phosphorus are precipitated in the present step, according tothe present inventors' findings, the content ratio of compoundsincluding phosphorus (hereinafter, also simply referred to as a“phosphorus compounds”) in a solid component precipitated immediatelyafter the start of increasing pH (hereinafter also referred to as “earlystage precipitate”) is higher, and the content ratio of phosphorus in asolid component precipitated later (hereinafter also referred to as“later stage precipitate”) is lower. Therefore, recovering the earlystage precipitate by performing a below-described step of recovering(Step S230) during the step of increasing pH can recover a solidcomponent with a higher phosphorus ratio separately from a solidcomponent with a lower phosphorus ratio.

The phosphorus compounds recovered from steelmaking slag can be reusedas a phosphorus source. Therefore, recovery of the solid component witha high content of the phosphorus compounds enables easy reuse ofphosphorus. In addition, while Ca compounds recovered from steelmakingslag can be reused as iron-making materials, produced iron would becomebrittle when the iron-making materials contain phosphorus compounds. Thecontent of the phosphorus compounds in the solid component for reuse asan iron-making material is thus preferably low. Therefore, when a solidcomponent with high phosphorus compound content and a solid componentwith low phosphorus compound content are separately obtained from a CO₂aqueous solution containing phosphorus and Ca, refining of the recoveredsolid component becomes easier or unnecessary, and also the quality ofproducts made from the recovered solid components can be furtherimproved.

During this procedure, most of phosphorus is precipitated before the pHof the CO₂ aqueous solution increases by 1.0. For further increasing thecontent ratio of phosphorus in the early stage precipitate and that ofcalcium in the later stage precipitate, the early stage precipitate isrecovered before the pH increases by preferably 1.0, more preferably0.6, and still more preferably 0.4.

The pH of a CO₂ aqueous solution can be increased by, for example,adding an alkaline substance into the CO₂ aqueous solution. Examples ofthe alkaline substances that can be fed into the CO₂ aqueous solutioninclude calcium hydroxide, ammonia and sodium hydroxide. Calciumhydroxide, ammonia or sodium hydroxide can be fed by dissolving the samein water, and adding the resultant solution to the CO₂ aqueous solution.Calcium hydroxide, ammonia and sodium hydroxide each may be a commercialproduct, or a substance contained in a liquid such as a waste fluid. Inthe case of adding calcium hydroxide contained in a waste fluid, a wastefluid generated during the production of acetylene by reacting, e.g.,calcium carbide with water can be added to the CO₂ aqueous solution. Theslag-immersed water provided by immersing steelmaking slag into water,the above described magnetic-separation water or hydration-treatmentwater may be added to the CO₂ aqueous solution. The slag-immersed water,magnetic-separation water and hydration-treatment water may be obtainedby subjecting the steelmaking slag from which calcium is to be recoveredor alternatively another steelmaking slag to immersion into water,magnetic separation or hydration treatment, respectively. Among thealkaline substances, calcium hydroxide, a waste fluid mostly composed ofcalcium hydroxide, the slag-immersed water, the magnetic-separationwater or the hydration-treatment water is preferred for use. Thesecontain mostly calcium hydroxide, and therefore using the same canreduce the amount of unnecessary substances, such as ammonia and sodium,mixed in the obtained calcium compounds. Unlike the use of ammonia andsodium hydroxide that remain in water, reprocessing of water after therecovery of calcium compounds becomes unnecessary.

Increasing the pH of a CO₂ aqueous solution also causes precipitationof, for example, Fe, Mn and P contained in the CO₂ aqueous solution insmall amounts, as the solid component. Accordingly, an aqueous solutionafter the recovery of Ca therefrom (former CO₂ aqueous solution) by themethod according to the present embodiment allows waste water treatmentto become simpler or unnecessary, thereby suppressing the waste watertreatment cost.

The aqueous solution after the recovery of Ca therefrom (former CO₂aqueous solution) contains substantially no metallic elements, such asCa, Fe, Mn, Al and P, and CO₂, and thus can be reused in the same step,thereby achieving no waste water.

For further increasing the recovery rate of Ca, removal of carbondioxide may be performed in combination with the increasing pH.

Recovery Step: Recovery of Solid Component

In the present step, a solid component precipitated in the precipitationstep (Step S220) is recovered (Step S230). The precipitated solidcomponent can be recovered by any conventional method, such as vacuumfiltration or pressure filtration. The solid component contains Caderived from steelmaking slag.

Hereinafter, the present invention will be described more specificallywith reference to Examples. However, these Examples do not limit thescope of present invention to the specific methods described below.

EXAMPLES Experiment 1

Steelmaking slag A and steelmaking slag B each having a component ratioas shown in Table 1 were prepared. The components of the steelmakingslag were measured by IPC optical emission spectrometry and chemicalanalysis. Steelmaking slag A and steelmaking slag B were pulverizedusing a hammer mill so that the average particle diameter thereof became100 μm or less. The average particle diameter of the pulverized slag wasconfirmed using a laser diffraction/scattering type particle sizedistribution measuring device.

TABLE 1 Steelmaking Component Ratio (mass %) Slug Ca Fe Si Mn Mg Al P A35.3 15.8 7.2 4.1 2.5 2.0 0.8 B 31.3 11.2 7.3 3.0 1.9 1.8 2.0

1-1. Magnetic Separation

Slag slurry was provided for each of steelmaking slag A and steelmakingslag B by dispersing the steelmaking slag into 50 times as much amountof water by mass ratio.

For each of steelmaking slag A and steelmaking slag B, a part of thepulverized steelmaking slag was subjected to the heat treatment in theair at 600° C. for 10 minutes. Subsequently, two portions of slag slurrywere provided for each of steelmaking slag A and steelmaking slag B bydispersing the steelmaking slag subjected to the heat treatment and thencooled to room temperature into 50 times as much amount of water by massratio.

Each obtained slag slurry was poured into the lower part of a drum typemagnetic separator, which has a permanent magnet fixed on the bottomhalf of the rotating drum thereof, and the rotating drum was rotated atthe peripheral speed of 3 m/min for 5 minutes. The slag magneticallyattracted to the rotating drum was then recovered from the top. Themagnetic flux density on the drum surface of the magnetic separator wasset to 0.03 T.

The Fe content in the recovered slag was measured by the ICPspectrometry and chemical analysis. The Fe recovery rate (%) of themagnetic separation was calculated by dividing the measured Fe contentin the recovered slag by the Fe content in the steelmaking slag used forthe magnetic separation (steelmaking slag A or steelmaking slag B).

1-2. Separation of Steelmaking Slag from Water

The slag slurry that was not magnetically attracted during the magneticseparation was separated into steelmaking slag and water by pressurefiltration.

As the separated water had a pH of 12.0 to 12.5, and thus was consideredto contain Ca eluted from the steelmaking slag, the amount of Cacontained in the separated water was measured by ICP spectrometry andchemical analysis. The elution rate (%) of Ca after the magneticseparation was calculated by dividing the measured Ca content in theseparated water by the Ca content in the steelmaking slag used for themagnetic separation (steelmaking slag A or steelmaking slag B).

For comparison, each of steelmaking slag A and steelmaking slag B wasdispersed in water, and separated into steelmaking slag and water bypressure filtration after 5 minutes. The amount of Ca contained in theseparated water was then measured by ICP spectrometry and chemicalanalysis. The elution rate (%) of Ca upon contact with water wascalculated by dividing the measured Ca content in the separated water bythe Ca content in the steelmaking slag used for the contact with water(steelmaking slag A or steelmaking slag B).

1-3. Contact With CO₂ Aqueous Solution

The steelmaking slag separated from the water was divided into two, andeach half was brought into contact with a CO₂ aqueous solution by one ofthe following two methods.

CO₂ contact 1 (without fracturing): 0.25 kg of the steelmaking slagseparated from water after the magnetic separation was mixed with 50 Lof water and stirred while carbon dioxide was blown at the flow rate of7 L/min at normal temperature (20 to 40° C.), thereby eluting Ca.

CO₂ contact 2 (with fracturing): a ball mill was charged with 0.25 kg ofthe steelmaking slag separated from water after the magnetic separation,50 L of water and 10 L (apparent volume) of pulverizing balls having aball diameter of 10 mm Subsequently, the ball mill was rotated at aperipheral speed of 70 m/min to fracture the steelmaking slag whilecarbon dioxide was blown at the flow rate of 7 L/min at normaltemperature (20 to 40° C.), thereby eluting Ca.

After 30 minutes, each steelmaking slag was separated from the CO₂aqueous solution, and the amount (kg/50 L) of Ca eluted into the CO₂aqueous solution was measured by ICP spectrometry and chemical analysis.The elution rate (%) of Ca upon contact with the CO₂ aqueous solutionwas calculated by dividing the measured Ca amount by a value obtainedfrom the mass of immersed steelmaking slag multiplied by the Cacomponent ratio in the steelmaking slag.

For comparison, steelmaking slag A or steelmaking slag B was dispersedin water and immediately separated into steelmaking slag and water bypressure filtration, the same treatment was performed on the separatedsteelmaking slag, and the Ca amount eluted into the aqueous solutioncontaining carbon dioxide and the elution rate of Ca were measured inthe same manner.

Table 2 shows the types of steelmaking slag, whether the heat treatmentand magnetic separation were performed, the Fe recovery rate in themagnetic separation, the Ca elution rate after the magnetic separation,the Ca elution rate upon contact with water, and the Ca elution rateupon contact with a CO₂ aqueous solution.

TABLE 2 Ca Elution Ca Elution Rate by CO₂ Rate by CO₂ Heating FeRecovery Ca Elution Ca Elution Contact 1 Contact 2 Steel- after Rate inRate after Rate upon (without (with making Pulver- Magnetic MagneticMagnetic Contact with Fracturing) Fracturing) Category Slug izingSeparation Separation (%) Separation (%) Water (%) (%) (%) Example 1 A —Yes 11 4 — 34 — Example 2 B — Yes 10 4 — 32 — Example 3 A — Yes 11 4 — —51 Example 4 A Yes Yes 19 4 — — 57 Example 5 A Yes Yes 26 4 — — 61Example 6 B — Yes 10 4 — — 50 Example 7 B Yes Yes 17 4 — — 58 Example 8B Yes Yes 24 4 — — 60 Comparative A — No — — 3 27 — Example 1Comparative B — No — — 4 26 — Example 2 Comparative A — No — — 3 — 43Example 3 Comparative B — No — — 3 — 40 Example 4

The magnetic separation of the slag slurry could have recovered 10% ormore of Fe. All the compounds recovered during the procedure had theconcentrations of Fe at 40 mass % or more, Mn at 5 mass % or more, andMg at 1 mass % or more, as measured by the ICP spectrometry and chemicalanalysis, and thus can be reused in an iron-making process.

Bringing the steelmaking slag subjected to the magnetic separation incontact with a CO₂ aqueous solution was capable of eluting a high ratioof Ca compared to bringing the steelmaking slag in contact with a CO₂aqueous solution without performing the magnetic separation. Adding theCa elution rate after the magnetic separation (and before the contactwith the CO₂ aqueous solution) to the above ratio gave a high recoveryrate of Ca in any case.

During the procedure, when air was blown into a liquid componentobtained by the solid-liquid separation after the contact with the CO₂aqueous solution, and further the pH of the liquid component wasincreased by adding thereto a liquid component (having high pH) obtainedby the solid-liquid separation after the magnetic separation, a calciumcarbonate compound containing phosphorus was precipitated until pH 8,and thereafter a calcium carbonate compound containing less phosphoruswas precipitated until pH 10. Measuring the amount of Ca precipitated bythese steps revealed that most of Ca in steelmaking slag A orsteelmaking slag B was recovered in the previous steps.

Subjecting steelmaking slag to the heat treatment before the magneticseparation increased both the Fe recovery rate in the magneticseparation and the Ca elution rate upon contact with a CO₂ aqueoussolution after the magnetic separation.

Experiment 2

Steelmaking slag A used in experiment 1 was prepared, and a part thereofwas subjected to the heat treatment as in experiment 1.

2-1. Magnetic Separation, Hydration Treatment, and Separation ofSteelmaking Slag from Water

Slag slurry from each of steelmaking slag A not subjected to the heattreatment and steelmaking slag A subjected to the heat treatment wasprepared as in experiment 1, and subjected to the magnetic separation asin experiment 1, followed by separation into steelmaking slag and waterby pressure filtration.

During the above procedure, hydration treatment was performed before,simultaneously with, or after the magnetic separation by one of thefollowing methods.

2-1-1. Immersion and Still Standing (After Magnetic Separation)

Slag slurry which was not recovered by the magnetic separation was leftto stand at room temperature for 60 minutes. Most of the steelmakingslag sank in 60 minutes.

2-1-2. Immersion and Stirring (Before Magnetic Separation)

Slag slurry was stirred for 60 minutes before the magnetic separationwhile ensuring that the steelmaking slag did not sink to the bottom andstay there. Steelmaking slag subjected to the heat treatment wassubjected to the hydration treatment after the heat treatment.

When performing both the immersion and stirring before the magneticseparation and the immersion and still standing after the magneticseparation, the immersion and stirring was performed for 20 minutes, andthe immersion and still standing for 40 minutes so that the total timeof the hydration treatment became 60 minutes.

2-1-3. Still Standing in Paste Form (After Magnetic Separation)

After the separation from water by the pressure filtration, thepaste-formed steelmaking slag was placed in a container and storedtherein for 60 minutes while ensuring that the steelmaking slag was notdried, and kept in a paste form.

2-1-4. Circulation (Simultaneously With Magnetic Separation)

The magnetic separation was performed by circulating slag slurry betweena magnetic separator and a tank where the slag slurry was stored beforebeing poured into the magnetic separator, for 30 minutes of rotationtime of the rotating drum. During the circulation, the circulation ratewas adjusted so that the flow rate of the slag slurry in the tank becamesatisfactorily slow.

2-2. Contact With CO₂ Aqueous Solution

Subsequently, each steelmaking slag was brought in contact with a CO₂aqueous solution by the CO₂ contact 2 (with fracturing) described inexperiment 1.

Table 3 shows whether the heat treatment was performed, the types ofhydration treatment performed before, simultaneously with, and after themagnetic separation, the Fe recovery rate in the magnetic separation,the Ca elution rate after the magnetic separation, and the Ca elutionrate upon contact with the CO₂ aqueous solution.

TABLE 3 Hydration Fe Recovery Ca Elution Ca Elution Hydration TreatmentHydration Rate in Rate after Rate by Heating Treatment (simultaneouslyTreatment Magnetic Magnetic CO₂ Contact 2 after (before Magnetic withMagnetic (after Magnetic Separation Separation (with Fracturing)Category Pulverizing Separation) Separation) Separation) (%) (%) (%)Example 11 — — — Immersion and 10 3 59 Still Standing Example 12 Yes — —Immersion and 24 3 70 Still Standing Example 13 Yes — — Still Standing24 4 69 in Paste Form Example 14 — Immersion and — — 11 4 56 StirringExample 15 Yes Immersion and — — 25 4 67 Stirring Example 16 YesImmersion and — Immersion and 26 4 69 Stirring Still Standing Example 17Yes — Circulation — 26 4 66

Bringing the steelmaking slag subjected to the magnetic separation andthe hydration treatment in contact with a CO₂ aqueous solution wascapable of eluting a high ratio of Ca into the CO₂ aqueous solutioncompared to experiment 1 that did not include the hydration treatment.Adding the Ca elution rate after the magnetic separation (and before thecontact with the CO₂ aqueous solution) to the above ratio also gave a Carecovery rate higher than those of experiment 1 in any case.

All the compounds recovered by the magnetic separation during theprocedure also had the concentrations of Fe at 40 mass % or more, Mn at5 mass % or more, and Mg at 1 mass % or more, as measured by the ICPspectrometry and chemical analysis, and thus can be reused in aniron-making process.

During the procedure as well, when air was blown into a liquid componentobtained by the solid-liquid separation after the contact with the CO ₂aqueous solution, and further the pH of the liquid component wasincreased by adding a liquid component (having high pH) obtained by thesolid-liquid separation after the magnetic separation, a calciumcarbonate compound containing phosphorus was precipitated until pH 8,and thereafter a calcium carbonate compound containing less phosphoruswas precipitated until pH 10. Measuring the amount of Ca precipitated bythese steps revealed that most of Ca in the steelmaking slag wasrecovered in the previous steps.

Also in this procedure, subjecting steelmaking slag to the heattreatment before the magnetic separation increased both the Fe recoveryrate in the magnetic separation and the Ca elution rate upon contactwith a CO₂ aqueous solution after the magnetic separation.

This application claims priority based on Japanese Patent ApplicationNo. 2017-006614, filed on Jan. 18, 2017, the entire contents of whichincluding the claims, the specification and the drawings areincorporated herein by reference.

INDUSTRIAL APPLICABILITY

The method according to the present invention for eluting Ca can easilyincrease the elution amount of Ca from steelmaking slag into an aqueoussolution containing carbon dioxide, as well as the recovery rate of Cafrom the steelmaking slag, and therefore is advantageous as a method forrecovering a Ca source in an iron-making process.

1. A method for eluting calcium from steelmaking slag, the methodcomprising: subjecting the steelmaking slag to magnetic separation toremove a compound containing iron from the steelmaking slag; andcontacting the steelmaking slag subjected to the magnetic separationwith an aqueous solution containing carbon dioxide.
 2. The methodaccording to claim 1, wherein the contacting includes contacting thesteelmaking slag with the aqueous solution containing carbon dioxidewhile fracturing or pulverizing the steelmaking slag, or grinding asurface of the steelmaking slag.
 3. The method according to claim 1,further comprising: subjecting the steelmaking slag to hydrationtreatment, and wherein the contacting includes contacting thesteelmaking slag subjected to the magnetic separation and the hydrationtreatment with the aqueous solution containing carbon dioxide.
 4. Themethod according to claim 3, wherein the hydration treatment isperformed before the magnetic separation.
 5. The method according toclaim 3, wherein the hydration treatment is performed after the magneticseparation.
 6. The method according to claim 3, wherein the hydrationtreatment is performed simultaneously with the magnetic separation. 7.The method according to claim 1, further comprising: subjecting thesteelmaking slag to heat treatment before performing the magneticseparation.
 8. A method for recovering calcium from steelmaking slag,the method comprising: eluting the calcium from the steelmaking slag bythe method according to claim 1; and recovering the eluted calcium.